Simple demonstration of using multiple panels with a single MouseListener

Main.java

package stackoverflow;

import java.awt.Color;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.GridLayout;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.border.LineBorder;

public class Main {
    public static void main(String[] args) {
        new Main();
    }
    
    public Main() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                JFrame frame = new JFrame();
                frame.add(new BoardPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }
    
    public class BoardPane extends JPanel {
        public BoardPane() {
            setLayout(new GridLayout(12, 12));
            
            MouseAdapter mouseHandler = new MouseAdapter() {
                private CellPane lastClicked;
                @Override
                public void mouseClicked(MouseEvent e) {
                    if (lastClicked != null) {
                        lastClicked.setBackground(null);
                    }
                    if (e.getSource() instanceof CellPane) {
                        CellPane cell = (CellPane) e.getSource();
                        cell.setBackground(Color.RED);
                        lastClicked = cell;
                    }
                }
            };
            for (int index = 0; index < 12 * 12; index++) {
                CellPane cell = new CellPane();
                add(cell);
                cell.addMouseListener(mouseHandler);
            }
        }        
    }
    
    public class CellPane extends JPanel {
        
        public CellPane() {
            setBorder(new LineBorder(Color.BLACK));
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(40, 40);
        }
    }
}